The goal of this project is to develop and validate assays to detect the presence of replication competent retroviruses (RCR) in preparations of viral vectors to be used for clinical purposes. The currently recommended procedure is to test 1% of the cells and 5% of the supernatant for the presence of RCR using the S+L- assay as a readout. The goal of this project is to assess the contribution of different parameters to relative sensitivity for RCR detection. In addition, we are developing a stock of RCR that will be available to others that is to be used as a standard in validation of assay sensitivity.